Solar System

The Solar System or solar system^[1] comprises the Sun and the retinue of celestial objects gravitationally bound to it: the eight planets, their 162 known moons,^[2] three currently identified dwarf planets and their four known moons, and thousands of small bodies. This last category includes asteroids, meteoroids, comets, and interplanetary dust.

In broad terms, the charted regions of the Solar System consist of the Sun (astronomical symbol ), four rocky bodies close to it called the inner planets, an inner belt of rocky asteroids, four giant outer planets and a second belt of small icy bodies known as the Kuiper belt. In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets are in turn orbited by natural satellites (usually termed "moons" after Earth's Moon) and every planet past the asteroid belt is encircled by planetary rings of dust and other particles. The planets other than Earth are named after gods and goddesses from Greco-Roman mythology.

From 1930 to 2006, Pluto, the largest known Kuiper belt object, was considered the Solar System's ninth planet. However, in 2006 the International Astronomical Union (IAU) created an official definition of the term "planet".^[3] Under this definition, Pluto is reclassified as a dwarf planet, and there are eight planets in the Solar System. In addition to Pluto, the IAU currently recognizes two other dwarf planets: Ceres, the largest object in the asteroid belt, and Eris, which lies beyond the Kuiper belt in a region called the scattered disc.

Layout and structure

The principal component of the Solar System is the Sun, a main sequence G2 star that contains 99.86% of the system's known mass and dominates it gravitationally.^[4] Jupiter and Saturn, the Sun's two largest orbiting bodies, account for more than 90% of the system's remaining mass.^[5] (The currently hypothetical Oort cloud would also hold a substantial percentage were its existence confirmed.)^[6]

Most objects in orbit around the Sun lie within the ecliptic, a shallow plane parallel to that of Earth's orbit. The planets are very close to the ecliptic while comets and kuiper belt objects are usually at significantly greater angles to it.

All of the planets and most other objects also orbit with the Sun's rotation; in a counter-clockwise direction as viewed from a point above the Sun's north pole. (There are exceptions to this rule, a notable one being Halley's comet.)

There is a direct relationship between how far away a planet is from the Sun and how quickly it orbits. Mercury, the closest to the Sun, travels the fastest, while Neptune, being much farther from the Sun, travels more slowly. Objects orbit in an ellipse around the Sun, so an orbiting object's distance from the Sun varies in the course of its year. Its closest approach to the Sun is known as its perihelion while its farthest point from the Sun is called its aphelion. Although the orbits of the planets are nearly circular (with perihelions roughly equal to their aphelions), many comets, asteroids and objects of the Kuiper belt follow highly elliptical orbits with large differences between perihelion and aphelion.

The paths of objects around the Sun travel according to a law of planetary motion discovered by German astronomer Johannes Kepler in the early 1600s. Under Kepler's laws, each planet orbits along an ellipse with the Sun at one focus of the ellipse. However, Newton's laws of motion dictate that just as the planets orbit around the Sun, so the Sun is minutely affected by the gravity of the planets, and moves in a much, much smaller eliptical trajectory about the focal point too. The primary focal point in the Solar System is that between the Sun and Jupiter, since Jupiter is far and away the largest of the planets. This point lies just outside the Sun itself and is roughly equivalent to the Solar System's centre of mass, or barycentre.^[7]

Astronomers most often measure distances within the solar system in astronomical units or AU. One AU is the approximate distance between the Earth and the Sun or roughly 149 598 000 km (93,000,000 mi). Pluto is roughly 39 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light year, the best known unit of interstellar distance, is roughly 63,240 AU.

Informally, the Solar System is sometimes divided into separate zones. The first zone, known as the inner Solar System, includes the four terrestrial planets and the main asteroid belt. The outer Solar System is sometimes defined as "everything beyond the asteroids". Alternatively, the term may be used to describe the region beyond Neptune, with the four gas giants considered a separate "middle zone".^[8]

One common misconception is that the orbits of the major objects within the Solar System (planets, Pluto and asteroids) are equidistant. To cope with the vast distances involved, many representations of the Solar System simplify these orbits by showing them the same distance apart. However, in reality, with a few exceptions, the Solar System is arranged so that the farther a planet or belt is from the Sun, the larger the distance between it and the previous orbit. For example, Venus is approximately 0.33 AU farther out than Mercury while Jupiter is 1.9 AU from the farthest extent of the asteroid belt and Neptune's orbit is roughly 20 AU farther out than that of Uranus. Attempts have been made to determine a correlation between these distances (see Bode's Law) but to date there is no accepted theory that explains the orbital distances.

Planets, dwarf planets, and small solar system bodies

In a decision passed by the International Astronomical Union General Assembly on 24 August 2006, the objects in the Solar System other than the Sun and natural satellites were divided into three separate groups: planets, dwarf planets and small solar system bodies.

Under this classification, a planet is any body in orbit around the Sun that a) has enough mass to form itself into a spherical shape and b) has cleared its immediate neighborhood of all smaller objects. Eight objects in the Solar System currently meet this definition; they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Dwarf planet was a second and new classification. The key difference between planets and dwarf planets is that while both are required to orbit the Sun and be of large enough mass that their own gravity pulls them into a nearly round shape, dwarf planets are not required to clear their neighborhood of other celestial bodies. Three objects in the solar system are currently included in this category; they are Pluto (formerly considered a planet), the asteroid Ceres, and the scattered disc object Eris. The IAU will begin evaluating other known objects to see if they fit within the definition of dwarf planets. The most likely candidates are some of the larger asteroids and several Trans-Neptunian Objects such as Sedna, Orcus, and Quaoar.

The remainder of the objects in the Solar System were classified as small solar system bodies (SSSBs). As the IAU noted in its resolution:

Formation

The current hypothesis of Solar System formation is the nebular hypothesis, first proposed in 1755 by Immanuel Kant and independently formulated by Pierre-Simon Laplace.^[10] The nebular theory holds that 4.6 billion years ago (a date determined via radiometric dating of meteorites),^[11] the Solar System formed from the gravitational collapse of a gaseous cloud. This initial cloud was likely several light-years across and probably played host to the births of several stars.^[12] Although the process was initially viewed as relatively tranquil, recent studies of ancient meteorites reveal traces of elements only formed in the hearts of very large exploding stars, indicating that the environment in which the Sun formed was within range of a number of nearby supernovae. The shock wave from these supernovae may have triggered the formation of the Sun by creating regions of overdensity in the surrounding nebula, causing gravitational forces to overcome their internal gas pressure, and thus, them in turn to collapse.^[13]

One of these regions of collapsing gas (known as the solar nebula) would form what became the Sun. This region had a diameter of between 7000 and 20,000 AU^[12][14] and a mass just over that of the Sun (by between 0.1 and 0.001 solar masses).^[15] As the nebula collapsed, conservation of angular momentum meant that it spun faster. As the material within the nebula condensed, the atoms within it began to collide with increasing frequency, causing them to release energy as heat. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc. ^[12] As the competing forces associated with gravity, gas pressure, magnetic fields, and rotation acted on it, the contracting nebula began to flatten into a spinning protoplanetary disk with a diameter of roughly 200 AU^[12] and a hot, dense protostar at the center.^[16][17]

Studies of T Tauri stars, young, pre-fusing solar mass stars believed to be similar to the Sun at this point in its evolution, show that they are often accompanied by discs of pre-planetary matter.^[15] These discs extend to several hundred AU and are rather cool, reaching only a thousand kelvins at their hottest.^[18]

Eventually, the temperature and pressure at the core of the Sun became so great that the hydrogen began to fuse, creating an internal source of energy which countered the force of gravitational contraction. At this point the Sun became a fully fledged star.

From the remaining cloud of gas and dust, the various planets formed. The currently accepted method by which the planets formed is known as accretion, in which the planets began as dust grains in orbit around the central protostar, which initially formed by direct contact into clumps between one and ten kilometres in diameter, which in turn collided to form larger bodies (planetesimals), of roughly 5 km in size gradually increasing by further collisions by roughly 15 cm per year over the course of the next few million years.^[19]

The inner solar system was too warm for volatile molecules like water and methane to condense, and so the planetesimals which formed there were relatively small (comprising only 0.6% the mass of the disc) ^[12] and composed largely of compounds with high melting points, such as silicates and metals. These rocky bodies eventually became the terrestrial planets. Farther out, the gravitational effects of Jupiter made it impossible for the protoplanetary objects present to come together, leaving behind the asteroid belt.^[20]

Farther out still, beyond the frost line, where more volatile icy compounds could remain solid, Jupiter and Saturn were able to gather more material than the terrestrial planets, as those compounds were more common. They became the gas giants, while Uranus and Neptune captured much less material and are known as ice giants because their cores are believed to be made mostly of ices (hydrogen compounds).^[21][22]

After 100 million years, the pressure and density of hydrogen in the centre of the collapsing nebula became great enough for the protosun to begin thermonuclear fusion, which increased until hydrostatic equilibrium was achieved.^[23] The young Sun's solar wind (see below) then cleared away all the gas and dust in the protoplanetary disk, blowing it into interstellar space, thus ending the growth of the planets. T-Tauri stars have far stronger stellar winds than more stable, older stars.^[24][25]

Sun

The Sun is the Solar System's parent star, and far and away its chief component. Its large mass gives it an interior density high enough to sustain nuclear fusion, releasing enormous amounts of energy, most of which is radiated into space in the form of electromagnetic radiation including visible light. It is classed as a moderately large yellow dwarf; however, this name is misleading, as on the scale of stars in our galaxy, the Sun is rather large and bright. Stars are classified based on their position on the Hertzsprung-Russell diagram, a graph which plots the brightness of stars against their surface temperatures. Generally speaking, the hotter a star is, the brighter it is. Stars which follow this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. This has led many astronomy textbooks to label the Sun as "average;" however, stars brighter and hotter than it are rare, whereas stars dimmer and cooler than it are common. The vast majority of stars are dim red dwarfs, though they are under-represented in star catalogues as we can observe only those few that are very near the Sun in space.^[26]

The Sun's position on the main sequence means, according to current theories of stellar evolution, that it is in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion, and been forced, as older red giants must, to fuse more inefficient elements such as helium and carbon. The Sun is growing increasingly bright as it ages. Early in its history, it was roughly 75 percent as bright as it is today.^[27] Calculations of the ratios of hydrogen and helium within the Sun suggest it is roughly halfway through its life cycle, and will eventually begin moving off the main sequence, becoming larger and brighter but also cooler and redder, until, about five billion years from now, it too will become a red giant.^[28]

The Sun is a population I star, meaning that it is fairly new in galactic terms, having been born in the later stages of the universe's evolution. As such, it contains more elements heavier than hydrogen and helium ("metals" in astronomical parlance) than older population II stars such as those found in globular clusters.^[29] Since elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, the first generation of stars had to die before the universe could be enriched with them. For this reason, the very oldest stars contain very few metals, while stars born later have more. This high metallicity is thought to have been crucial in the Sun's developing a planetary system, because planets form from accretion of metals.^[30]

Interplanetary medium

The Sun radiates a continuous stream of charged particles, a plasma known as solar wind, ejecting it outwards at speeds greater than 2 million kilometres per hour,^[31] creating a very tenuous atmosphere (the heliosphere), that permeates the solar system for at least 100 AU. This environment is known as the interplanetary medium. The influence of the Sun's rotating magnetic field on the interplanetary medium creates the largest structure in the solar system, the heliospheric current sheet.^[32]

Earth's magnetic field protects its atmosphere from interacting with the solar wind. However, Venus and Mars do not have magnetic fields, and the solar wind causes their atmospheres to gradually bleed away into space.^[33]

The interplanetary medium is home to at least two disclike regions of cosmic dust. The first, which lies in the inner solar system, is known as the zodiacal dust cloud and is responsible for the phenomenon of zodiacal light. It was likely formed by collisions within the asteroid belt brought on by interactions with the planets.^[34] The second, which extends from about 10 AU to about 40 AU, was probably created by similar collisions within the Kuiper belt.^[35][36]

Inner planets

The four inner or terrestrial planets are characterised by their dense, rocky composition, few or no moons, and lack of ring systems. They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and possess tectonic surface features such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets which are closer to the Sun than the Earth is (i.e. Mercury and Venus).

The four inner planets are:

Mercury

Mercury (0.4 AU), the closest planet to the Sun, is also the smallest of the planets, at only 0.055 Earth masses. Mercury is very different from the other terrestrial planets; it has no natural satellite, and its only known geological features besides impact craters are "wrinkle ridges" probably produced by a period of contraction early in its history.^[37] Its almost negligible atmosphere consists of atoms blasted off its surface by the solar wind.^[38] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun's energy.^[39][40]

Venus

Venus (0.7 AU) is of comparable mass to the Earth (0.815 Earth masses), and, like Earth, possesses a thick silicate mantle around an iron core, as well as a substantial atmosphere and evidence of internal geological activity, such as volcanoes. However, it is much drier than Earth and its atmosphere is 90 times as dense. Venus has no natural satellite. It is the hottest planet, with surface temperatures over 400 °C, most likely due to the amount of greenhouse gases in the atmosphere.^[41] Although no definitive evidence of current geological activity has yet been detected on Venus, its substantial atmosphere and lack of a magnetic field to protect it from depletion by the solar wind suggest that it must be regularly replenished by volcanic eruptions.^[42]

Earth

The largest and densest of the inner planets, Earth (1 AU) is also the only one to demonstrate unequivocal evidence of current geological activity. Earth is the only planet known to have life. Its liquid hydrosphere, unique among the terrestrial planets, is probably the reason Earth is also the only planet where plate tectonics has been observed, because water acts as a lubricant for subduction.^[43] Its atmosphere is radically different from the other terrestrial planets, having been altered by the presence of life to contain 21 percent free oxygen.^[44] It has one satellite, the Moon; the only large satellite of a terrestrial planet in the Solar System.

Mars

Mars (1.5 AU), at only 0.107 Earth masses, is smaller than Earth and Venus. It possesses a tenuous atmosphere of carbon dioxide. Its surface, peppered with vast volcanoes and rift valleys such as Valles Marineris, shows that it was once geologically active and recent evidence^[45] suggests this may have been true until very recently. Mars possesses two tiny moons (Deimos and Phobos) thought to be captured asteroids.^[46]

Asteroid belt

Asteroids are mostly small solar system bodies that are composed in significant part of rocky and metallic non-volatile minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be the remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter. Asteroids range in size from hundreds of kilometers to as small as dust. All asteroids save the largest, Ceres, are classified as small solar system bodies; however, a number of other asteroids, such as Vesta and Hygieia, could potentially be reclassed as dwarf planets if it can be conclusively shown that they have achieved hydrostatic equilibrium. The asteroid belt contains tens of thousands - and potentially millions - of objects over one kilometre in diameter.^[47] However, despite their large numbers, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth.^[48] In contrast to its various depictions in science fiction, the main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10^-4 m are called meteoroids.^[49]

Ceres

Ceres (2.77 AU) is the largest astronomical body in the asteroid belt and the only known dwarf planet in this region. It has a diameter of slightly under 1000 km, large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in the nineteenth century, but was reclassified as an asteroid in the 1850s as further observation revealed additional asteroids.^[50] It was again reclassified in 2006, and is now considered to be a dwarf planet.

Asteroid groups

Asteroids in the main belt are subdivided into asteroid groups and families based on their specific orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets^[51] which may have been the source of Earth's water.

Trojan asteroids are located in either of Jupiter's L₄ or L₅ points, (gravitationally stable regions leading and trailing a planet in its orbit) though the term is also sometimes used for asteroids in any other planetary Lagrange point as well. Hilda asteroids are those Trojans whose orbits are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.

The inner solar system is also dusted with rogue asteroids, many of which cross the orbits of the inner planets.

Outer planets

The four outer planets, or gas giants, (sometimes called Jovian planets) are so large they collectively make up 99 percent of the mass known to orbit the Sun. Jupiter and Saturn are true giants, at 318 and 95 Earth masses, respectively, and composed largely of hydrogen and helium. Uranus and Neptune are both substantially smaller, being only 14 and 17 Earth masses, respectively. Their atmospheres contain a smaller percentage of hydrogen and helium, and a higher percentage of “ices”, such as water, ammonia and methane. For this reason some astronomers suggested that they belong in their own category, “Uranian planets,” or “ice giants.” ^[52] All four of the gas giants exhibit orbital debris rings, although only the ring system of Saturn is easily observable from Earth. The term outer planet should not be confused with superior planet, which designates those planets which lie outside Earth's orbit (thus consisting of the outer planets plus Mars).

Jupiter

Jupiter (5.2 AU), at 318 Earth masses, is 2.5 times the mass of all the other planets put together. Its composition of largely hydrogen and helium is not very different from that of the Sun. Jupiter's strong internal heat creates a number of semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. The four largest of its 63 satellites, Ganymede, Callisto, Io, and Europa (the Galilean satellites) share elements in common with the terrestrial planets, such as volcanism and internal heating.^[53] Ganymede, the largest satellite in the Solar System, has a diameter larger than Mercury.

Saturn

Saturn (9.5 AU), famous for its extensive ring system, has many qualities in common with Jupiter, including its atmospheric composition, though it is far less massive, being only 95 Earth masses. Two of its 56 moons, Titan and Enceladus, show signs of geological activity, though they are largely made of ice.^[54] Titan, like Ganymede, is larger than Mercury; it is also the only satellite in the solar system with a substantial atmosphere.

Uranus

Uranus (19.6 AU) at 14 Earth masses, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt lies at over ninety degrees to the ecliptic. Its core is remarkably cold compared with the other gas giants, and radiates very little heat into space.^[55] Uranus has 27 satellites, the largest being Titania, Oberon, Umbriel, Ariel and Miranda.

Neptune

Neptune (30 AU), though slightly smaller than Uranus, is denser at 17 Earth masses, and radiates more internal heat than Uranus, but not as much as Jupiter or Saturn.^[56] Neptune has 13 moons. The largest, Triton, is geologically active, with geysers of liquid nitrogen,^[57] and is the only large satellite to revolve around its host planet in a retrograde motion. Neptune possesses a number of Trojan asteroids.

Comets

Comets are small solar system bodies, usually only a few kilometres across, composed largely of volatile ices and possessing highly eccentric orbits; generally having a perihelion within the orbit of the inner planets and an aphelion far beyond Pluto. When a comet approaches the Sun, its icy surface begins to sublimate, or boil away, creating a coma; a long tail of gas and dust which is often visible with the naked eye.

There are two basic types of comet: short-period comets, with orbits less than 200 years, and long-period comets, with orbits lasting thousands of years. Short-period comets, such as Halley's Comet, are believed to originate in the Kuiper belt, while long period comets, such as Hale-Bopp (pictured), are believed to originate in the Oort Cloud. Some comets with hyperbolic orbits may originate outside the solar system.^[58] Old comets that have had most of their volatiles driven out by solar warming are often categorized as asteroids.^[59]

Kuiper belt

The area beyond Neptune, often referred to as the outer solar system or simply the "trans-Neptunian region", is still largely unexplored.

This region's first formation is the Kuiper belt, a great ring of debris similar to the asteroid belt, but composed mainly of ice and far greater in extent, extending between 30 and 50 AU from the Sun. This region is thought to be the place of origin for short-period comets, such as Halley's comet. Though it is composed mainly of small solar system bodies, many of the largest Kuiper belt objects, such as Quaoar, Varuna, 2003 EL61, 2005 FY9 and Orcus, could soon be reclassified as dwarf planets. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km; however, the total mass of the Kuiper belt is relatively low, perhaps only a tenth or even a hundredth the mass of the Earth.^[60] Many Kuiper belt objects have multiple satellites and most have orbits that take them outside the plane of the ecliptic.

The Kuiper belt can be roughly divided into two regions: the "resonant" belt, consisting of objects whose orbits are in some way linked to that of Neptune (orbiting, for instance, twice for every three Neptune orbits, or once for every two), which actually begins within the orbit of Neptune itself, and the "classical" belt, consisting of objects that don't have any resonance with Neptune, and which extends from roughly 39.4 AU to 47.7 AU.^[61] Members of the classical Kuiper belt are classified as Cubewanos, after the first of their kind to be discovered, 1992 QB1.^[62]

Pluto and Charon

Pluto (39 AU average), is the largest known object in the Kuiper belt and was previously accepted as the smallest planet in the Solar System. In 2006, it was reclassified as a dwarf planet by the Astronomers Congress organized by the International Astronomers Union (IAU).^[63] Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU at aphelion. Prior to the 2006 redefinitions, Charon was considered a moon of Pluto, but in light of the redefinition it is unclear whether Charon will continue to be classified as a moon of Pluto or as a dwarf planet itself. Charon does not exactly orbit Pluto in a traditional sense; Charon is about one-tenth the mass of Pluto and the center of gravity of the pair is not within Pluto. Both bodies orbit a barycenter of gravity above the surface of Pluto (in empty space), making Pluto-Charon a binary system. Two much smaller moons, Nix and Hydra, orbit Pluto and Charon. Pluto lies in the resonant belt, having a 3:2 resonance with Neptune (ie, it orbits twice round the Sun for every three Neptune orbits). Those Kuiper belt objects which share this orbit with Pluto are called Plutinos.^[64]

Scattered disc

Overlapping the Kuiper belt but extending much further outwards is the scattered disc. Scattered disc objects are believed to have been originally native to the Kuiper belt, but were ejected into erratic orbits in the outer fringes by the gravitational influence of Neptune's early outward migration. Most scattered disc objects have perihelia within the Kuiper belt but aphelia as far as 150 AU from the Sun. Their orbits are also highly inclined to the ecliptic plane, and are often almost perpendicular to it. Some astronomers, such as Kuiper belt co-discoverer David Jewitt, consider the scattered disc to be merely another region of the Kuiper belt, and describe scattered disc objects as "scattered Kuiper belt objects."^[65]

Eris

Eris (68 AU average) is the largest known scattered disc object and was the cause of the most recent debate about what constitutes a planet since it is at least 5% larger than Pluto with an estimated diameter of 2400 km (1500 mi). It is now the largest of the known dwarf planets.^[66] It has one moon, Dysnomia.

The object has many similarities with Pluto: its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and is steeply inclined to the ecliptic plane, at 44 degrees, more so than any known object in the solar system except the newly-discovered object 2004 XR₁₉₀ (also known as "Buffy"^[67]) and is believed to consist largely of rock and ice.^[68]

Centaurs

The Centaurs, which roughly extend from 9 to 30 AU, are icy comet-like bodies that orbit in the region between Jupiter and Neptune. The largest known Centaur, 10199 Chariklo, has a diameter of between 200 and 250 km.^[69] The first centaur to be discovered, 2060 Chiron, has been called a comet since it has been shown to develop a coma just as comets do when they approach the sun.^[70] Some astronomers class Centaurs as scattered Kuiper belt objects along with the residents of the scattered disc, merely Kuiper belt objects scattered inward, rather than outward.^[71]

Farthest regions

The point at which the solar system ends and interstellar space begins is not precisely defined, since its outer boundaries are delineated by two separate forces: the solar wind and the Sun's gravity. The solar wind extends to a point roughly 130 AU from the Sun, whereupon it surrenders to the surrounding environment of the interstellar medium. The Sun's gravity however, holds sway to almost halfway to the next star system. The vast majority of the solar system therefore, is completely unknown; however, recent observations of both the solar system and other star systems have led to an increased understanding of what is or may be lying at its outer edge.^[72]

Heliopause

The heliosphere expands outward in a great bubble to about 95 AU, or three times the orbit of Pluto. The edge of this bubble is known as the termination shock; the point at which the solar wind collides with the opposing winds of the interstellar medium. Here the wind slows, condenses and becomes more turbulent, forming a great oval structure known as the heliosheath that looks and behaves very much like a comet's tail; extending outward for a further 40 AU at its stellar-windward side, but tailing many times that distance in the opposite direction. The outer boundary of the sheath, the heliopause, is the point at which the solar wind finally terminates, and one enters the environment of interstellar space.^[73] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.^[74]

Sedna

Sedna is a large, reddish Pluto-like object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 928 AU at aphelion and takes 12,050 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper Belt as it has too distant a perihelion to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, one which also may include the object 2000 CR₁₀₅, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3420 years.^[75] Sedna is very likely a dwarf planet, though its shape has yet to be determined with certainty.

Oort cloud

The theorized Oort cloud, is a great mass of up to a trillion icy objects that is believed to be the source for all long-period comets and to surround the solar system like a shell from 50,000 to 100,000 AU beyond the Sun. It is believed to be composed of comets which were ejected from the inward Solar System by gravitational interactions with the outer planets. Because the Sun's gravitational hold on them is so weak, Oort cloud objects move only very slowly, though they can be perturbed by such rare events as collisions, or the gravitational effects of a passing star or the galactic tides.^[76]

Galactic context

The solar system is located in the Milky Way galaxy, a barred spiral galaxy with a diameter estimated at about 100,000 light years containing approximately 200 billion stars. Our Sun resides in one of the Milky Way's outer spiral arms, known as the Orion Arm or Local Spur.^[77] While the orbital speed and radius of the galaxy are not accurately known, estimates place the solar system at between 25,000 and 28,000 light years from the galactic center and its speed at about 220 kilometres per second, completing one revolution every 225-250 million years. This revolution is known as the Solar System's galactic year.^[78]

The solar system appears to have a very remarkable orbit. It is both extremely close to being circular, and at nearly the exact distance at which the orbital speed matches the speed of the compression waves that form the spiral arms. Evidence suggests that the Solar System has remained between spiral arms for most of the existence of life on Earth. The radiation from supernovae in spiral arms could theoretically sterilize planetary surfaces, preventing the formation of complex life, save perhaps in the deepest oceans. The solar system also lies well outside the star-crowded environs of the galactic centre. The opposing gravitational tugs from so many close stars within the galactic centre would have prevented planets from forming.^[79]

The Solar apex, the direction of the Sun's path through interstellar space, is near the constellation of Hercules in the direction of the current location of the bright star Vega.^[80] At the galactic location of the solar system, the escape velocity with regard to the gravity of the Milky Way is at least 500 km/s.^[81]

Neighborhood

The immediate galactic neighborhood of the Solar System is known as the Local Interstellar Cloud or Local Fluff; an area of dense cloud in an otherwise sparse region known as the Local Bubble, an hourglass-shaped cavity in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma that suggests it is the product of several recent supernovae.^[82]

There are relatively few stars within ten light years (95 trillion km) of the Sun. The closest is the triple star system Alpha Centauri, which is located roughly 4.4 light years away (the outlying star of the triple, the red dwarf Proxima Centauri, is closer, at 4.22 light years). Alpha Centauri A and B are a closely tied pair of Sun-like stars. The stars next closest to the Sun are the red dwarfs Barnard's Star (at 6 light years), Wolf 359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest star within ten light years is Sirius, a bright blue dwarf star roughly twice the Sun's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light years away. The remaining systems within ten light years are the binary red dwarf system UV Ceti (8.7 light years) and the solitary red dwarf Ross 154 (9.7 light years).^[83] Our closest solitary sunlike star is Tau Ceti, which lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only 60 percent its luminosity.^[84]

Extrasolar systems

For many years, the Solar System was the only known example of planets in orbit around a star. The discovery in recent years of many extrasolar planets has led to the term "solar system" being applied generically to all the newly discovered systems. Technically, however, it should strictly refer to Earth's system only, as the word "solar" is derived from the Sun's Latin name, Sol. Other such systems are usually referred to by the names of their parent star (for example "the Alpha Centauri system" or "the 51 Pegasi system").

Recent studies of extrasolar systems neighboring Earth's have shown that our system's configuration might not be common, as the vast majority so far discovered have been found to be markedly different. For instance, many extrasolar planetary systems contain a "hot Jupiter";^[85] a planet of comparable size to Jupiter that nonetheless orbits very close to its star, at, for instance, 0.05 AU. It has been hypothesised that while the giant planets in these systems formed in the same place as the gas giants in Earth's solar system did, some sort of migration took place which resulted in the giant planet spiralling in towards the parent star. Any terrestrial planets which had previously existed would presumably either be destroyed or ejected from the system. On the other hand, the apparent prevalence of hot Jupiters could result from a sampling error, as planets of similar size at greater distances from their stars are more difficult to detect.^[86]

Discovery and exploration

For many thousands of years, people, with a few notable exceptions, did not believe the Solar System existed. The Earth was believed not only to be stationary at the centre of the universe, but to be categorically different from the divine or ethereal objects that moved through the sky. While Nicholas Copernicus and his predececessors, such as the Indian mathematician-astronomer Aryabhatta and the Greek philosopher Aristarchus of Samos, had speculated on a heliocentric reordering of the cosmos, it was the conceptual advances of the 17th century, led by Galileo Galilei, Johannes Kepler, and Isaac Newton, which led gradually to the acceptance of the idea not only that Earth moved round the Sun, but that the planets were governed by the same physical laws that governed the Earth, and therefore could be material worlds in their own right, with such earthly phenomena as craters, weather, geology, seasons and ice caps.

Telescopic observations

The first exploration of the solar system was conducted by telescope, when astronomers first began to map those objects too faint to be seen with the naked eye.

Galileo Galilei was the first to discover physical details about the individual bodies of the Solar System. He discovered that the Moon was cratered, that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it.^[87] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn.^[88] Giovanni Domenico Cassini later discovered four more moons of Saturn, the Cassini division in Saturn's rings, and the Great Red Spot of Jupiter.^[89]

Edmund Halley realised in 1705 that repeated sightings of a comet were in fact recording the same object, returning regularly once every 75-6 years. This proved once and for all that comets were not atmospheric phenomena, as had been previously thought, and was the first evidence that anything other than the planets orbited the Sun.^[90]

In 1781, William Herschel was looking for binary stars in the constellation of Taurus when he observed what he thought was a new comet. In fact, its orbit revealed that it was a new planet, Uranus, the first ever discovered.^[91]

Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars and Jupiter that was initially considered a new planet. However, subsequent discoveries of thousands of other small worlds in the same region led to their eventual separate reclassification: asteroids.^[92]

By 1846, discrepancies in the orbit of Uranus led many to suspect a large planet must be tugging at it from farther out. Urbain Le Verrier's calculations eventually led to the discovery of Neptune.^[93]

Further discrepancies in the orbits of the planets led Percival Lowell to conclude yet another planet, "Planet X" must still be out there. After his death, his Lowell Observatory conducted a search, which ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto was, however, found to be too small to have disrupted the orbits of the outer planets, and its discovery was therefore coincidental. Like Ceres, it was initially considered to be a planet, but after the discovery of many other similarly sized objects in its vicinity it was eventually reclassified as a dwarf planet.^[93]

In 1992, astronomers David Jewitt of the University of Hawaii and Jane Luu of the Massachusetts Institute of Technology discovered 1992 QB1. This object proved to be the first of a new population, which came to be known as the Kuiper Belt; an icy analogue to the asteroid belt of which such objects as Pluto and Charon were deemed a part.^[94][95]

Mike Brown, Chad Trujillo and David Rabinowitz announced the discovery of Eris in 2005, a Scattered disc object larger than Pluto and the largest object discovered in orbit round the Sun since Neptune.^[96]

Observations by spacecraft

Since the start of the space age, a great deal of exploration has been performed by unmanned space missions that have been organized and executed by various space agencies. The first probe to land on another solar system body was the Soviet Union's Luna 2 probe, which impacted on the Moon in 1959. Since then, increasingly distant planets have been reached, with probes landing on Venus in 1965, Mars in 1976, the asteroid 433 Eros in 2001, and Saturn's moon Titan in 2005. Spacecraft have also made close approaches to other planets: Mariner 10 passed Mercury in 1973.

The first probe to explore the outer planets was Pioneer 10, which flew by Jupiter in 1973. Pioneer 11 was the first to visit Saturn, in 1979. The Voyager probes performed a grand tour of the outer planets following their launch in 1977, with both probes passing Jupiter in 1979 and Saturn in 1980 – 1981. Voyager 2 then went on to make close approaches to Uranus in 1986 and Neptune in 1989. The Voyager probes are now far beyond Neptune's orbit, and astronomers anticipate that they will encounter the heliopause which defines the outer edge of the solar system in the next few years.^[73][97]

All planets in the solar system have now been visited to varying degrees by spacecraft launched from Earth, the last being Neptune in 1989. Through these unmanned missions, humans have been able to get close-up photographs of all of the planets and, in the case of landers, perform tests of the soils and atmospheres of some.

No Kuiper belt object has been visited by a spacecraft. Launched on 19 January 2006, the New Horizons probe is currently enroute to becoming the first man-made spacecraft to explore this area. This unmanned mission is scheduled to fly by Pluto in July 2015. Should it prove feasible, the mission will then be extended to observe a number of other Kuiper belt objects.^[98]